The Organic Light Emitting Display (OLED) device possesses many outstanding properties of self-illumination, low driving voltage, high luminescence efficiency, short response time, high clarity and contrast, near 180° view angle, wide range of working temperature, applicability of flexible display and large scale full color display. The OLED is considered as the most potential display device.
The OLED can be categorized into two major types according to the driving ways, which are the Passive Matrix OLED (PMOLED) and the Active Matrix OLED (AMOLED), i.e. two types of the direct addressing and the Thin Film Transistor matrix addressing. The AMOLED comprises pixels arranged in array and belongs to active display type, which has high lighting efficiency and is generally utilized for the large scale display devices of high resolution.
The OLED display element generally comprises a substrate, an anode located on the substrate, a Hole Injection Layer located on the anode, a Hole Transporting Layer located on the Hole Injection Layer, an emitting layer located on the Hole Transporting Layer, an Electron Transport Layer located on the emitting layer, an Electron Injection Layer located on the Electron Transport Layer and a Cathode located on the Electron Injection Layer. The principle of the OLED element is that the illumination generates due to the carrier injection and recombination under the electric field driving of the semiconductor material and the organic semiconductor illuminating material. Specifically, the Indium Tin Oxide (ITO) electrode and the metal electrode are respectively employed as the anode and the cathode of the Display. Under certain voltage driving, the Electron and the Hole are respectively injected into the Electron and Hole Transporting Layers from the cathode and the anode. The Electron and the Hole respectively migrate from the Electron and Hole Transporting Layers to the Emitting layer and bump into each other in the Emitting layer to form an exciton to excite the emitting molecule. The latter can illuminate after the radiative relaxation.
Recently, the tandem organic electroluminescent element is gaining more and more attention from the academia and the industry with its excellent current efficiency, luminance and working life. The tandem organic electroluminescent element generally comprises a plurality of light emitting units, and the plurality of light emitting units are connected by a charge generation layer, and the light emissions thereof do not affect with each other. The biggest advantage of the tandem organic electroluminescent element is that its brightness and current efficiency can be greatly improved. The existing research results showed that the brightness and the current efficiency of the tandem organic electroluminescent element are directly proportional to the number of layers.
However, similar to the single layer organic electroluminescent element, the tandem organic electroluminescent element also has the problem of life attenuation. The lifetime attenuation of the tandem organic electroluminescent element generally includes the lifetime attenuation of the respective light emitting units and the lifetime attenuation of the charge generation layer. The lifetime attenuation of the charge generation layer will further cause the attenuation of the luminance of one or more of the light emitting units, and consequently, result in a larger color shift after the prolonged use of the tandem organic electroluminescent element.